This invention relates to subsurface scale inhibitor squeeze treatments. More particularly, a solution of a scale inhibitor having a solubility that significantly decreases with decreasing pH is injected into a subsurface formation adjacent a production well and thereafter the pH of the solution is lowered to precipitate the scale inhibitor in the formation.
Production of water, oil and gas is frequently hampered by deposition of scales, for example, calcium carbonate, calcium sulfate, barium sulfate, magnesium carbonate, magnesium sulfate, and strontuim sulfate. Scale deposition may result in reservoir damage, stuck subsurface pumps, plugged casing perforations, and plugged tubing and other subsurface equipment. Scale may also interfere with the operation of surface flowlines and other surface equipment through which the produced fluids pass and/or treated.
Sometimes scale problems may be relieved by changes in mechanical and other operational changes, but in many situations, it is necessary to resort to treatment with chemical scale inhibitors. During the past decade a great deal of research has provided increased knowledge of the way scale inhibitors work. Scale deposition is a complex crystalline process where the crystallizing mineral exceeds its solubiilty limit. This can be caused by a number of factors such as pressure and temperature changes, agitation, mixing of fluids and the like. After supersaturation occurs, nucleation takes place with initial formation of a precipitate or insoluble phase of the crystal forming mineral. This process can occur spontaneously or be promoted by the presence of already formed scale crystals or presence of other extraneous insoluble material such as sand particles, corrosion products or imperfections on surfaces. Scale ions contact a nuclei of very small size and absorb or become incorporated into the nuclei in a manner such that the crystal grows in a crystalline pattern. When this crystalline growth process is interfered with, crystalline growth of scale deposition is altered and even prevented.
Scale inhibitors function at threshold levels, for example, less than 10 to 50 ppm, by inhibiting crystal growth and adherence of the scale. The inhibitor, therefore, must be present during scale nucleation so that the growth sites are immediately positioned by the inhibitor. Evidence of this has been abundantly provided by comparing crystals formed in the presence and absence of a scale inhibitor. This means that the scale inhibitor must be in the scale forming water prior to the change or mixing or other phenomenon, for example, pressure or temperature drops near the formation face or casing perforations, can cause scale deposition. To prevent scale deposition at the formation face or in the casing perforations, the scale inhibitor must be present in the formation and be slowly dissolved or desorbed from the rock surface into the produced formation waters on an uninterrupted basis in the produced formation waters that form scale. To accomplish these results, frequently scale inhibitors are pumped or squeezed into the formation. Sufficient inhibitor is squeezed at sufficient frequency to dissolve enough scale inhibitor in the produced water to satisfy the needs of the system being treated. Numerous publications and patents discuss scale and its prevention or alleviation, for example, patent Nos. 3,704,750 and 3,827,977 which describe scale inhibitor squeeze techniques.
Typical scale inhibitors are phosphates, phosphonates, polymaleic acids, polymers and phosphate esters. Some squeeze techniques rely on physical adsorption of the scale inhibitor onto the formation matrix rock. As formation waters are produced in a treated well, the inhibitor is desorbed from the rock matrix and feeds back with the produced fluids. But adsorption squeeze procedures result in limited squeeze life.
In addition to adsorption, inhibitor may be retained in the formation during a squeeze treatment by precipitation. In this process, the inhibitor is precipitated within the interstices of the formation. When the well is returned to production, the scale inhibitor precipitate redissolves slowly over an extended period of time to provide the necessary active inhibition as waters are produced into the wellbore. Scale inhibitor squeezes relying on precipitation typically use scale inhibitors that are precipitated by divalent cations. Generally, the acidic scale inhibitor solution, with or without acid retarders, is squeezed into the formation where the acid usually reacts with calcium carbonate in the formation producing divalent calcium ions. The divalent cations combine with the scale inhibitor and cause it to precipitate. Such precipitation in the matrix increases scale inhibitor retention and thereby increases treatment life. In addition, to the acid and calcium carbonate reaction, the divalent cations needed to precipitate the scale inhibitor may be naturally occurring in the reservoir brine, ion-exchanged from the reservoir rock or injected into the formation with the inhibitor. Unfortunately, inherent in these typical precipitation processes is the competition between scale formation and scale inhibitor precipitation. Therefore, if care is not taken in these squeeze applications, the formation near the well can be severly damaged.
It is an object of this invention to provide an inhibitor squeeze technique that does not depend on divalent cations to precipitate the scale inhibitor. It is a further object of this invention to describe a scale inhibitor squeeze process wherein scale inhibitor is precipitated in a subsurface water, gas or oil producing formation in a manner that is less likley to damage the conductivity of the producing formation. It is still a further object of this invention to provide a scale inhibiting formation squeeze method that may be repeated when scale inhibitor return concentration falls below necessary threshold concentrations over and over again without increasing formation damages by inhibitor precipitation.